So, normally this would indicate that the tunnel source and destination could not complete at the N-VDS specified MTU. This is pretty common in new builds because the actual MTU check doesn't occur until the tunnel is needed.
It'd be really cool if we could have some kind of "post-implementation network test"!
Anyhow, for the tunnel that is failing you want to run a vmkping against it with the DNF bit set and MTU set to 1600+:
vmkping -d -I vmk10 -s 1600 <the other end>
https://kb.vmware.com/s/article/1003728
